Your grasp of language is terrible
Look, everyone has talents. Not everyone is a talented typist. Heck, not every roleplayer is a talented typist. But there's a long road from being unable to spell certain things correctly and spewing about sentences that are incomprehensible masses of letters vaguely jammed into words.
You don't need to have your fingers dance across the keys like Sean Connery is watching over you and shouting vaguely helpful slogans, but you do need to be able to write clearly enough for people to know what in the world you mean. Roleplaying is already a complicated process of suspending disbelief, and trying to decipher incomprehensible non-sentences slows everything to a crawl while ramping up the frustration.
Eventually, people are just going to stop interacting with you because it's much less irritating in the long run.
Fortunately, this is one of the easiest things to fix. Occasionally dropping a capital or misspelling something is hardly a big deal; you just need to strike a better balance between speed and comprehensibility. Oddly, this is a problem I have never encountered with roleplayers for whom English is a second language, so make of that what you will.
Your character isn't fun to be around
This is something I've touched on in the past with jerks and points related, but it bears repeating because there are a lot of characters that just aren't fun to be around. I've made a few myself. Playing someone who's not fun to be around is going to mean that people don't want your character around, even if you're fine as a person.
Since roleplaying runs on drama and explosions, this is not the usual definition of "fun" that I'm using here. More often than not you've got a character who either actively negates interesting things or one with a very rigid and demonstrably irritating way of interacting with the world. Or both. You make players and characters uncomfortable, meaning that players look for a reason for their characters to leave and then don't return.
One of the nicer things that I've seen the Final Fantasy XIV community do is offer a light form of character workshopping to the community as a whole -- you can explain your character concept and get feedback on what makes sense, what doesn't, and what might cause problems. This is invaluable if you're having this issue because obviously there's a reason people don't want to play with you. Ask for feedback, and take that into account. Don't just ask the people who still put up with your character; ask the people who don't. Yes, there might be some personal invective in there, but if everyone says that your character is unpleasant for the same reasons, there's probably an underlying truth to it.
Your character assaults the lore
I have no problem bending the lore. Truth be told, I actively encourage it in little places; as long as you fit the feel of the world, you should be able to introduce new elements. But there's a difference between the margins and the main story. There's a reason Lord of the Rings Online puts you on a side mission alongside the Fellowship rather than having players burst through the front gate of Mordor on a motorcycle in the first five minutes while fighting off a horde of deathbots, largely due to the non-existence of both motorcycles and deathbots in the setting.
So it's really unsurprising that no one wants to play with your half-Vulcan half-elven cyborg Sith lord in that same game. Or that no one wants to talk with your character who claims to be the love child of Bilbo and Gollum because no and also so much more no. You aren't just breaking the lore; you are assaulting it in ways that should have you tried for crimes.
Calm down and dial it back a bit. Remember that the reason people are roleplaying in this game is that they enjoy the game and they like the setting. Outside of Champions Online, that setting has certain rules for what does and does not exist, and you need to adhere to those. If you don't want to compromise those elements of the character, ask yourself -- do they say anything interesting, or are they just there to make your character more special? The latter isn't a valid reason to break the setting.
People are just jerks
Sometimes people just don't want to roleplay with you because they're cliquish tools. Not often, but you should be aware of the possibility. I'm putting this last on the list because it does happen, but it should also be your last possible explanation when all else fails. Start by assuming that it's something you can fix.
As always, feedback is welcome down in the comments or via mail to firstname.lastname@example.org. Next week, let's talk trigger warnings and touchy subjects. The week after that? Making a living.
Every Friday, Eliot Lefebvre fills a column up with excellent advice on investing money, writing award-winning novels, and being elected to public office. Then he removes all of that, and you're left with Storyboard, which focuses on roleplaying in MMOs. It won't help you get elected, but it will help you pretend you did. If you need a refresher, check out the Storyboard Library.